The function views_ui_build_form_url() has been renamed to views_ui_build_form_path() in Backdrop 1.0.0.

Drupal 7:
  $form['type'] = array(
    '#type' => 'select',
    '#title' => t('Formatter'),
    '#options' => $formatters,
    '#default_value' => $this->options['type'],
    '#ajax' => array(
      'path' => views_ui_build_form_url($form_state),
    ),
    '#submit' => array(
      'views_ui_config_item_form_submit_temporary',
    ),
    '#executes_submit_callback' => TRUE,
  );
Backdrop CMS:
$form['type'] = array(
    '#type' => 'select',
    '#title' => t('Formatter'),
    '#options' => $formatters,
    '#default_value' => $this->options['type'],
    '#ajax' => array(
      'path' => views_ui_build_form_path($form_state),
    ),
    '#submit' => array('views_ui_config_item_form_submit_temporary'),
    '#executes_submit_callback' => TRUE,
  );
Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers